High intensity lamp with cermet igniter

ABSTRACT

A high intensity metal vapor electric discharge lamp having self-heating main electrodes at opposite ends and a starter electrode at one end. A cermet igniter comprising a sintered mixture of metal and refractory or emissive metal oxide is attached to and spaced close to the main electrode at the end having the starter electrode. The igniter operates as much as 1,000*C. cooler than the main electrode and retains its emission material throughout life. A glow or arc initiated on it readily transfers to the main electrode. It serves to reduce the starting voltage or prevent rise in starting voltage in seasoned lamps.

nite States Patent [191 hite et al.

[111 3,840,768 51 Oct. 8, 1974 HIGH INTENSITY LAMP WITH CERMET IGNITER[73] Assignee: General Electric Company,

Schenectady, NY.

22 Filed: Aug. 10,1972 21 Appl. No.2 279,548

[52] U.S. CI. 313/208, 313/198, 313/213 [51] Int. Cl. H01j 17/32 [58]Field of Search 313/184, 208, 211, 198; 315/60 {5 6] References CitedUNITED STATES PATENTS 1,840,055 l/l932 Rentschler....; 313/208 2,259,94710/1941 Blackburn 313/198 X 2,749,467 6/1956 Rigden 313/184 Green 315/60Olson et al 313/208 X Primary Examiner-Ronald L. Wibert AssistantExaminer-Richard A. Rosenberger Attorney, Agent, or FirmErnest W.Legree; Henry P. Truesdell; Frank L. Neuhauser 57] ABSTRACT A highintensity metal vapor electric discharge lamp having self-heating mainelectrodes at opposite ends and a starter electrode at one end. A cermetigniter comprising a sintered mixture of metal and refractory oremissive metal oxide is attached to and spaced close to the mainelectrode at the end having the starter electrode. The igniter operatesas much as 1,000C. cooler than the main electrode and retains itsemission material throughout life. A glow or are initiated-on it readilytransfers to the main electrode. It serves to reduce the startingvoltage or prevent rise in starting voltage in seasoned lamps.

8 Claims, 2 Drawing Figures HIGH INTENSITY LAMP WITH CERMET IGNITERBACKGROUND OF THE INVENTION The invention relates to metal vapor arclamps using an arc discharge in metal or metal halide vapors to producelight, and is particularly concerned with the starting of such lamps.

High pressure metal vapor lamps such as mercury or metal halide lampsgenerally have self-heating main electrodes at opposite ends andareusually provided with an auxiliary starting electrode. It'may take theform of a starter wire spaced close to one of the main electrodes. Thelatter is usually negative, or cathode, at the a.c. phase moment whenignition occurs, so that it is convenient to speak of this mainelectrode adjacent to the auxiliary electrode as cathode, and of theother more distant main electrode as anode. After closing the circuit,when the starter voltage becomes sufficiently positive a glow dischargeappears between starter and cathode. This glow discharge quicklytransfers to the anode from the starter, and subsequently is convertedinto an are, such completing the ignition process.

The main electrodes comprise a base metal body bearing emissionmaterial, the form commonly used being a close wound tungsten coilbearing low work function emission mix in the interstices between turns.Presence of the emission material tends to facilitate ignition of theglow discharge or Paschen breakdown and also transition of the glow intoan arc. However, operation of the lamp with repeated starts tends toremove the emission material by processes such-as ion bombardment andsimple flaking due to thermal expansion or distortion. Concomitantly, itis observed that some lamps become hard starters as life progresses.

SUMMARY OF THE INVENTION 'We have reasoned that the development of hardstarting in discharge lamps and particularly in metal halide lamps mayresult from loss of emission material from electrodes in service. Thepotential hazard isthe rate may be low, and it can be accelerated bywanv dering of the cathode terminus or by split spot mode of burning. y

In accordance with our invention, the development of hard starting withlife is prevented by assuring that there is emission mix available toprovide normal starting throughout life. This is achieved by providingan igniter having a well-bonded supply of emission mix in a region outof range of the normal wandering of the arc terminus and maintained at atemperature several hundred degrees Centigrade lower than the front endof the main electrode.

In a preferred embodiment of the invention, a ceramic-metal or cermetigniter comprising a sintered mixture of metal and metal oxide on aconductor is attached to the main electrode at the end of the lamphaving the auxiliary electrode. The metal oxide may be refractory, oremissive, or preferably refractory and emissive. A desirable arrangementis to locate the igniter in the space between the main electrode and theauxiliary electrode. A glow or arc initiated at the cermet igniter canreadily transfer to the cathode. The end of the cermet igniter is placedback from the tip or face of the cathode so thatthe arc, which prefers ashort length, will quickly transfer to the cathode. The cermet igniteroperates at least l,000C. cooler than the main electrode and retains itsemission material throughout life. Thus it serves to reduce the startingvoltage or prevent rise in starting voltage in seasoned lamps.

DESCRIPTION OF DRAWING In the drawing:

FIG. -1 shows in side view a metal halide lamp embodying the invention.

FIG. 2 is an enlarged sectioned view of the cathode" electrode providedwith cermet igniter.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to FIG. 1 of the drawing,a mercury metal halide vapor arc lamp 1 in which the invention isembodied comprises an outer vitreous envelope or jacket 2 of ellipsoidalform having a neck portion 3. The neck is closed by 'a re-entrant stem 4having a press 5 through which extend stiff inlead wires 6,7. Theseinleads are connected at their outer ends to the contacts of the usualscrew-type base 8, namely the threaded shell 9 and the insulated centercontact 10.

The inner arc tube 12 made of quartz or fused silica has therein atopposite ends a pair of main arcing electrodes, 13 at the base end and14 at the dome end, plus an auxiliary starting electrode 15 at the baseend. The electrodes are supported on inleads which include intermediatethin molybdenum foil sections 16 hermetically sealed to the flattenedends 17, 18 of the arc tube, commonly referred to as full diameter pinchseals. The main electrodes 13, 14 each comprise a double layer tungstenwire helix 19 (best seen in FIG. 2) wrapped around a tungsten core wire20. The mainelectrodes are activated by thorium oxide which coats theturns and fills the interstices within the helix.

The are tube is supported within the outer jacket by a divided ortwo-part mount. The lower mount sectionv comprises U-shaped support rod21 welded to inlead 6 and having metal straps 22 extending across itsends to clamp pinch seal 11. The upper mount section comprises invertedU-shaped support rod 23 to which is attached a springy collar 24engaging a re-entrant dimple 25 in the dome end of the outer envelope,and which has straps 26 extending across its ends and clamping pinchseal 18. The right angle braces 27 connected to support rods 21 and 23engage blind notches 28 in the ends of pinch seals 17, 18 to stiffen theassembly.

Main electrode 13 is connected to inlead 6 through strap 29 and thelower support rod, while main electrode 14 is connected to inlead 7through curving wire 31. Starting electrode 15 is connected to inlead 7through a current limiting resistor 32 which may have a value forinstance of 40,000 ohms. A thermal switch consisting of a U-shapedbimetal 33 is welded at one end to the inlead of main electrode 13 withits free end disposed to engage the inlead of starting electrode 15 asthe lamp warms up. The auxiliary electrode is thereby connected to theadjacent main electrode during operation of the lamp whereby to preventharmful electrolytic effects in accordance with the teachings of U.S.Pat. No. 3,226,597 to Green.

The arc tube contains an inert rare gas such as argon at a low pressure,for instance 25 torr, to facilitate starting and warm-up, and a fill ofmetal or metal halide. By way of example the arc tube may contain aquantity of mercury which is substantially totally vaporized and exertsa partial pressure in the range of l to 15 atmospheres during operation.Additionally there may be contained a metal halide such as sodium iodidein excess of the quantity vaporized at the operating temperature, plussmaller amounts of other metal halides such as thallium and indiumiodides, or scandium iodide.

In accordance with our invention, the availability of emission mix fornormal starting is assured by providing a well-bonded supply of it in aregion out of range of the normal wandering of the arc terminus. In theillustrated embodiment best seen in FIG. 2, such supply takes the formof a ceramic or cermet igniter 35. The igniter comprises a sinteredmixture of refractory metal and emissive or refractory metal oxidepowders as a coating 36 on a refractory metal wire 37 which is welded tocore wire 20 of main electrode 13 below or to the rear of helix 19. Thepreferred mixture for a metal halide containing lamp is tungstenpowder'and thoria, each 50 percent by weight. The mixture, in a flexiblesleeve of polyvinyl chloride, is pressed at 40,000 pounds per squareinch around an 18 mil diameter tungsten wire. The sleeve is dissolvedaway and the green compact is then fired in vacuo for an hour attemperatures of 1,750 to 2,000C. After firing, the thickness of thecermet coating 36 around wire 37 averaged approximately 6 mils. Othermixtures which have been used include tungsten and alkaline earthoxides; tungsten, thoria, and rare earth oxides; and tungsten, tungstencarbide, and thoria.

The tip or upper end of the igniter 35 is preferably placed back fromthe tip or front face of electrode 13 in order that the are, whichprefers a short path, will quickly transfer to the cathode, that is, toelectrode 13. The cap shape of the cermet coating 36 is not critical andwire support 37 may project through the coating without harm.

The tip of auxiliary starter electrode 15 is shown in dash lines in FIG.2 to indicate its location relative to the igniter. Ignition starts as aglow discharge between starter wire 15 and igniter 35. The dischargetransfers without delay from starter to anode" l4 and from igniter tocontiguous cathode" l3 and developes into an arc. Operating temperatureof the cermet igniter, about 900C, is at least 1,000 Centigrade belowthat of the upper coil region of the main electrode, so the igniter doesnot dispense emission mix as does the electrode. Also sputtering due tospot motion, routine on the main electrode, is absent from the igniter.

In lamps provided with igniters according to the invention, moderatereduction in voltage required to start at room temperature was observedin lamps seasoned less than 1 hour; the reduction became moresubstantial, about 7 percent, after more starts and longer seasoning.This is shown in Table 1 below for lamps containing mercury and metaliodides. The term fast cycle" denotes a start including 2 seconds ofburning followed by 28 seconds off for accelerated seasoning.

TABLE 1 Average RMS Starting Voltages At 50 At 20F. Control With ControlWith Lamps lgniters Lamps lgniters After 1 hour or less of seasoning 237221 237 230 After 20 starts and 100 hours seasoning 220 207 247 233After 220 starts and 300 hours seasonin 220 203 247 237 After 220 startsan 300 hours seasoning and 8000 fast cycles" 203 I 233 217 There is alsoan appreciable reduction, up to 6%, at lower temperatures (-20F.) wherestarting is often problematical.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A high intensity metal vapor lamp comprising:

a vitreous envelope containing metal vapor and having a pair ofthermionic self-heating arc-supporting main electrodes sealed therein atopposite ends and a starter electrode at one end;

and an igniter connected to the main electrode at said one end, saidigniter comprising a sintered mixture of metal and metal oxide on aconductor, said igniter being spaced close to the main electrode towhich it is connected whereby it operates at a lower temperature withoutappreciable loss of metal oxide and a discharge initiated on it readilytransfers to said main electrode.

2. A lamp as in claim 1 wherein the metal oxide in said mixture isrefractory or emissive oxide.

3. A lamp as in claim 1 wherein said igniter comprises a sinteredmixture of tungsten powder and refractory emissive metal oxide on atungsten conductor.

4. A lamp as in claim 1 wherein said igniter comprises a sinteredmixture of tungsten powder and thorium oxide on a tungsten conductor.

5. A high intensity metal vapor lamp comprising:

a vitreous envelope containing an inert starting gas,

mercury and metal halide,

a pair of thermionic self-heating arc-supporting main electrodes sealedinto opposite ends of said envelope and a starter electrode at one end,each of said main electrodes comprising a winding of tungsten wire on atungsten core activated by emissive metal oxide thereon,

and an igniter connected to the main electrode at said one end, saidigniter comprising a sintered mixture of tungsten powder and emissivemetal oxide powder on a tungsten conductor attached for the tungstencore of said main electrode to the rear of the winding of tungsten wire,

said igniter being spaced close to said main electrode to which it isconnected, whereby it operates at a lower temperature withoutappreciable loss of emissive metal oxide and a discharge initiated on itreadily transfers to said main electrode.

6. A lamp as in claim 5 wherein said igniter operates at a temperatureat least several hundred degrees Centigrade lower than the front end ofthe main electrode.

said main electrode.

1. A high intensity metal vapor lamp comprising: a vitreous envelopecontaining metal vapor and having a pair of thermionic self-heatingarc-supporting main electrodes sealed therein at opposite ends and astarter electrode at one end; and an igniter connected to the mainelectrode at said one end, said igniter comprising a sintered mixture ofmetal and metal oxide on a conductor, said igniter being spaced close tothe main electrode to which it is connected whereby it operates at alower temperature without appreciable loss of metal oxide and adischarge initiated on it readily transfers to said main electrode.
 2. Alamp as in claim 1 wherein the metal oxide in said mixture is refractoryor emissive oxide.
 3. A lamp as in claim 1 wherein said ignitercomprises a sintered mixture of tungsten powder and refractory emissivemetal oxide on a tungsten conductor.
 4. A lamp as in claim 1 whereinsaid igniter comprises a sintered mixture of tungsten powder and thoriumoxide on a tungsten conductor.
 5. A high intensity metal vapor lampcomprising: a vitreous envelope containing an inert starting gas,mercury and metal halide, a pair of thermionic self-heatingarc-supporting main electrodes sealed into opposite ends of saidenvelope and a starter electrode at one end, each of said mainelectrodes comprising a winding of tungsten wire on a tungsten coreactivated by emissive metal oxide thereon, and an igniter connected tothe main electrode at said one end, said igniter comprising a sinteredmixture of tungsten powder and emissive metal oxide powder on a tungstenconductor attached for the tungsten core of said main electrode to therear of the winding of tungsten wire, said igniter being spaced close tosaid main electrode to which it is connected, whereby it operates at alower temperature without appreciable loss of emissive metal oxide and adischarge initiated on it readily transfers to said main electrode.
 6. Alamp as in claim 5 wherein said igniter operates at a temperature atleast several hundred degrees Centigrade lower than the front end of themain electrode.
 7. A lamp as in claim 5 wherein the emissive metal oxidein said sintered mixture is thorium oxide.
 8. A lamp as in claim 5wherein said igniter is located between said starter electrode and themain electrode to which it is connected and projects forward less thansaid main electrode.